The FBI has investigated more than 100 suspected Islamic extremists within the U.S. military community, according to an NPR report.

About a dozen of the cases were considered serious, according to the report.

The suspects were those who allegedly demonstrated a strong intent to attack military targets -- individuals that FBI and Department of Defense officials call “insider threats,” according to NPR.

At a closed Congressional hearing, officials said the suspects could be active and reserve military personnel or individuals who have military facilities access, such as contractors and family members with dependent ID cards, NPR reported.

"I know one can say that as a percentage of the millions of people in active military service or working with contractors, the numbers you talk about are a small percentage of the total, but the reality is it only took one man, Nidal Hasan, to kill 13 people at Fort Hood and injure a lot more,” Sen. Joseph Lieberman, chairman of the Senate Committee on Homeland Security, told NPR.

Hasan, an Army major, is charged with attacking soldiers at Fort Hood in November 2009.